This invention relates to a method of controlling material hardening using a laser beam, and the associated laser beam hardening device. The inventive method and device are capable of correctly measuring and controlling the temperature of a portion of material to be hardened, while scanning the laser over the material in an oscillating motion, and controlling the laser output power level from one hardening cycle to a next.
A method of hardening workpiece along a path with a predetermined width, by spatially oscillating the laser beam in a direction intersecting a feeding direction has been recently proposed. It is necessary correctly to detect the temperature of a portion to be hardened and to control the temperature of the portion to obtain proper hardening.
Such temperature control is not conventionally executed in a laser beam hardening device. It is possible to envision a method of measuring the temperature of a portion of material to be hardened, by non-contact temperature sensing using an infrared sensor or the like. But it is difficult using such a sensor to trail the point at which the laser energy is applied. The method is not practical because the torch device for applying the laser beam energy has a complicated track along which the laser beam is oscillated spatially, namely scanning at least partly lateral to the feeding direction or path along which hardening is to proceed, over a length and width as well as to some three-dimensional hardening depth.
When hardening workpiece by oscillating the laser beam spatially (feeding along a path while scanning the beam at least partly laterally of the feeding direction), the velocity of the laser beam over the material surface differs, resulting in differences in the amount of energy applied per unit area. A workpiece might be hardened by radiation for a given time over some arbitrary width of scanning amplitude and feed rate, with corresponding variation in the area to which the beam energy was applied. Greater coverage area requires greater beam power to achieve the same applied radiation per unit of material area, if the feed and scanning amplitude are constant, and the laser beam power output is constant, the radiation energy per unit area of material still is not constant because of periodic variations in the scan velocity. The radiation applied per unit area may be too high at phase positions where the relative velocity of the beam is low, as to melt the workpiece locally. On the other hand, the radiation energy per unit area may be too low at positions where the relative velocity of the beam is high, as to fail to raise the temperature of the workpiece to the level desired for hardening. Proper hardening is not possible unless the beam power and the scan velocity together achieve the desired material temperature.
A method and device for controlling hardening with a laser beam are needed, capable of properly controlling the radiation beam energy level applied by the laser beam, in coordination with the scanning amplitude of the laser beam, so as to obtain a proper hardening action.
Additionally, the laser beam hardening device should be capable of correctly measuring the temperature of the material portion being hardened, even if the laser or torch moved in a complicated scanning action as mentioned above, and controlling the temperature of the portion is desired to be provided.